The field of STED microscopy (for which see DE 44 16 558 C1 and DE 100 63 276 A1), two lasers are usually used, i.e. one to excite a sample region and another to generate the stimulated emission. Generating the stimulated emission, in particular, requires high light power levels and at the same time the greatest possible flexibility in wavelength selection. Optically parametric oscillators (OPOs) are often used for this. For efficient frequency conversion, it is useful to pump an OPO optically with a high-power laser. These are usually mode-coupled pulsed lasers, which as a rule are very expensive. There are also costs for the excitation light source, which normally also comprises a mode-coupled pulsed laser. All the lasers must furthermore be exactly aligned in order to strike the individual sample regions exactly. In the case of pulsed excitation, it is important to ensure that the light pulses generating the stimulated emission arrive within a specific time frame (which depends on the lifetime of the excited states of the sample material) after the excitation light pulses. Since the two laser sources are independent of one another, exact synchronization of the emitted laser pulses is necessary. Synchronization of the pulsed lasers with one another is usually achieved by means of an active control system. This is complex, and often unsatisfactory and unstable in operation.
German Patent Application DE 100 56 382 discloses a light source for illumination in scanning microscopy, and a scanning microscope. The light source and the scanning microscope contain an electromagnetic energy source that emits light of one wavelength, and a means for spatial division of the light into at least two partial light beams. An intermediate element for wavelength modification is provided in at least one partial light beam. The advantage of this arrangement is that the light pulses in the two partial light beams are always synchronized with one another. It is disadvantageous, however, that two partial light beams must be generated, thereby reducing the intensity in each of the partial beams. Since the power level of the electromagnetic energy source, which for example can be a Ti:sapphire laser, is split, insufficient power is often available for the application.
A passive synchronization of pulse trains coming from two different lasers is disclosed, for example, in W. Seitz et al., Opt. Lett. Vol. 27, No. 6, p. 454 (2002). This describes a method and an arrangement for passively synchronizing two completely different lasers (in the specific case a Ti:sapphire laser and a Nd:YVO4 laser) with one another by optically modulating the intra-resonator losses of a slave laser (here the Nd:YVO4 laser) by means of a master laser (here the Ti:sapphire laser). A Fabry-Perot mirror, comprising a semiconductor whose energy band boundary lies between the photon energies of the two lasers, is used for this. This method is particularly simple and practical compared to other synchronization methods. The document gives no indication, however, that the system can be used as a light source for microscopic examinations.